Question

If a DNA strand has 45 cytosines, how many adenines does it have?

Answer 1

DNA is a double helical structure consisting of two strands made up of nitrogen bases attached with each other with the help of double or triple hydrogen bonds, phosphates, and deoxyribose sugar. But out of 4 types of bases 2 pairs with the other 2 and that is fixed.

Correct answer:
If a DNA strand has 45 cytosines, it will have 45 guanines because in DNA strands cytosine (C) of one strand always pairs with the guanine (G) of another strand. Similarly, the adenine (A) of one strand always pairs with the thymine (T) of another strand. So, the number of cytosines in a strand of DNA does not tell about the number of adenines present in that strand of DNA.

Additional Information:
DNA is made up of nucleotide molecules. Every molecule of nucleotide contains one phosphate group, one sugar group, and one nitrogen base.
The four types of nitrogen bases present in a nucleotide are adenine (A), thymine (T), guanine (G), and cytosine (C). The order of these bases determines the DNA's instructions or genetic code.
The nucleotides are attached to each other to form two long strands of DNA that spiral with each other to create a structure called a double helix. So, DNA is said to be the double-helical structure.
If we think of the double helix structure of DNA as a ladder, the phosphate and sugar molecules would be the sides of the ladder, whereas the bases would be the rungs of the ladder.
The bases on one strand pair with the bases on another strand meaning adenine on one strand pairs with the thymine of another strand, and similarly the guanine of one strand pairs with the cytosine of the opposite strand.
The bond between adenine and thymine may be a hydrogen covalent bond whereas the bond between guanine and cytosine is that hydrogen triple bond.
The DNA molecules are so good that they can not fit into cells without the proper packaging. To fit inside cells, DNA is coiled tightly to make structures called chromosomes.

Note:
In DNA strands cytosine (C) of one strand always pairs with the guanine (G) of another strand. Similarly, the adenine (A) of one strand always pairs with the thymine (T) of another strand.
There is a double hydrogen bond between adenine and thymine and a triple hydrogen bond between guanine and cytosine.
If we think of the double helix structure of DNA as a ladder, the phosphate and sugar molecules would be the sides of the ladder, whereas the bases would be the rungs of the ladder.